This invention relates, generally, to a high power current control device and more particularly to current limiting as well as static VAR generation by utilizing a superconducting coil.
Power distribution systems have always required the use of current limiting and current interrupting devices within the system so that short circuit fault conditions can be isolated. For this problem current interruption devices such as circuit breakers are generally utilized. However, devices such as circuit breakers are relatively slow in operation in that for AC systems several cycles of fault current are generally present before the circuit breaker trips out. Current limiting devices are particularly useful where it is necessary to limit short circuit current for at least a few cycles in an alternating current system or for a predetermined period of time in direct current systems so as to limit the stresses associated with high short circuit currents and enable circuit breakers to perform their functions. Many current limiting methods utilize series chokes or coils as an impedance to limit the rate of rise of fault current. However, in high power distribution systems the insertion of a conventional choke or coil would cause unacceptable power losses due to the resistance introduced by the windings of a choke or coil. With the introduction of superconductors a number of newer systems have utilized superconductive coil elements since the resistive losses associated with superconductors are orders of magnitudes lower than those in standard chokes or coils. Such types of devices may be found in U.S. Pat. No. 4,117,524, "Current-Limiting Devices", issued Sept. 26, 1978 to Parton et al.; U.S. Pat. No. 4,045,823, "Current Limiting Devices for Alternating Current Systems", issued Aug. 30, 1977 to Parton; U.S. Pat. No. 3,925,707, "High Voltage Current Limiting Circuit Breaker Utilizing a Superconductive Resistive Element", issued Dec. 9, 1975 to Bhate et al.; and U.S. Pat. No. 3,703,664, "Fault Current Limiter Using Superconductive Element", issued Nov. 21, 1972 to Cronin.
However, the above-mentioned patents either require that the superconductor be part of a saturable reactor core biasing scheme and, therefore require additional components such as reactor cores, or that they lose their superconductivity during excess current conditions or initiation of a switching operation to insert resistive elements. Moreover, the above-mentioned patents do not provide means for or envisage power factor control on a relatively rapidly changing basis to accommodate different load conditions.
It is desirable to have a device which utilizes a superconducting coil to limit fault current without requiring that the superconductor be driven out of saturation into a normal condition or require the superconductor to switch other resistive components into the circuit. It is also desirable to have a device which may exercise VAR generator control in conjunction with a superconductor. Such a scheme is taught in the present invention.
Briefly stated, a current control device is used for limiting and controlling current in an electrical transmission system. An electrical power source is used with a plurality of unidirectional devices connected with at least one superconducting impedance device so that current produced by a current generator is caused to flow in the superconductor in one direction only. A current generating device is connected to the superconducting impedance device for circulating a current in the superconducting impedance device. The unidirectional voltage device and the superconducting impedance device is connected with the power source so that the portion of current introduced by the power source which exceeds the value of the current produced by a current generator is impeded by inductance in the superconducting device.